FIG. 1 illustrates a conventional refrigeration system 10 (i.e. a freezer) typical of a use point to which the invention may be applied. A conveyor belt 12 is included on which material 13 to be refrigerated is transported. Conveyor belt 12 is positioned within freezer compartment 14 and has a variable speed drive that is user-controllable. A liquid cryogen (e.g., nitrogen) is sprayed onto product 13 through a number of nozzles mounted on manifolds 16 positioned along the path of belt 12 which in the example illustrated in FIG. 1, is moving from right to left. Sufficient nitrogen is sprayed into freezer compartment 14 to hold the temperature therein at a set point, using a temperature controller and control valve. Fans 18 are placed throughout freezer compartment 14 to circulate the gas atmosphere. A vent fan 20 discharges the nitrogen gas outside of the building.
The temperature of product 13 is typically measured every 30 minutes to assure that it falls within an acceptable range. After the periodic reading is taken, the internal freezer temperature, and sometimes the speed of belt 12, are adjusted in an attempt to hold product 13 within a preset temperature range. Typical residence times in freezer compartment 14 are from 3 to 30 minutes and the time to measure delivered product temperature is 10 or more minutes. Therefore, any change made to the internal temperature within freezer compartment 14 is based on conditions that existed some 13 to 40 minutes previously. For these reasons, it is necessary to hold the operating parameters within freezer compartment 14 as constant as possible. Those parameters include:
(1) condition and temperature of the inlet liquid nitrogen PA1 (2) temperature of the incoming product 13; PA1 (3) spacing of product 13 on the belt 12; PA1 (4) speed of circulating fans 18; PA1 (5) speed of belt 12; and PA1 (6) discharge rate of vent fan 20.
With the exception of the temperature of the inlet liquid nitrogen, all of these parameters are within the control of the operator. Thus, it is important that the refrigeration system include a means for controlling the cryogenic liquid nitrogen introduced into freezer compartment 14.
Liquid nitrogen is typically piped to freezer compartment 14 at temperatures between -301.degree. F. and -309.degree. F. which represents a three percent variation in refrigeration value. Liquid nitrogen droplets that are sprayed on the product furiously boil in flight, cooling the bulk of the droplets to -320.degree. F. Gas generated in this cooling process emerges at -320.degree. F. and becomes component A of the freezer atmosphere as shown in FIG. 1. The remaining portion of the liquid nitrogen droplet lands on the product and continues to boil, resulting in a high heat transfer rate. Gas generated in this boiling process also emerges at -320.degree. F. and becomes component B of the freezer atmosphere. The last component (C) of the freezer atmosphere is air-infiltration from the freezer input and output openings. Fans 18 enhance forced convection heat transfer from product 13 and have their speeds set as high as possible to achieve maximum heat transfer rates, but below a speed that will blow product 13 off belt 12.
Because the temperature within the freezer compartment is related to convection heat transfer, as the incoming nitrogen temperature increases, more nitrogen has to be boiled to cool itself and less is available to refrigerate the product. However, the total cold gas volume and temperature available for forced convection remains constant.
In FIG. 2, a spray bar 30 is illustrated that includes a pair of manifolds 32 which communicate with a plurality of nozzles 34. Liquid nitrogen is introduced into manifolds 32 via inlet 35 and exits through nozzles 34 towards product 13 on belt 12 as illustrated in FIG. 1. Typically, thirty or more nozzles 34 are used to spread the spray area across the width of belt 12. Because heat transfer in this area represents at least half of the total refrigeration, it is imperative that liquid nitrogen output from nozzles 34 be maintained constant and continuous.
In FIG. 3, a plot of flow from nozzles 34 versus distance along spray bar 30 illustrates that the nozzles closer to inlet 35 produce larger flow rates than nozzles near the extremities of manifolds 32. A number of factors affect the relative discharge rate at each of nozzles 34. Manifolds 32 are exposed to the freezer atmosphere and heat is transferred to the liquid nitrogen at a fairly constant rate per unit length along manifolds 32 As a result, the temperature of the liquid nitrogen increases as it travels through manifolds 32. The temperature rise is exacerbated by the fact that liquid flow is less in each segment of manifolds 32 between successive nozzles. Therefore, heat absorbed per pound of nitrogen is geometrically higher in each successive segment. As a result, the temperature and vapor pressure also increases geometrically at each nozzle Further, liquid delivered from each nozzle 34 is inversely proportional to the heat content of the nitrogen at inlet 35.
The result of the above factors on distribution of flow from nozzles 34 is shown in the chart of FIG. 4 which plots flow against nozzle position along manifolds 32. Curve 40 plots the fall-off in flow at a vapor pressure of 15; curve 42 at a vapor pressure of 17; and curve 44 at a vapor pressure of 19. As is known to those skilled in the art, a higher vapor pressure is illustrative of a higher temperature nitrogen. Note that curve 44 shows that nozzle F in FIG. 2 is completely shut off from flow as a result of the increased temperature of the nitrogen. Thus a relatively small change in vapor pressure at inlet 35 effectively shuts off nozzle F and possibly further nozzles that reside closer toward inlet 35. If the vapor pressure (i.e., temperature) of nitrogen entering inlet 35 can be maintained at a constant level, appropriate spray patterns can be maintained along the entire length of manifolds 32. However, liquid nitrogen that is supplied from a reservoir tank exhibits temperature variations that occur (1) as a result of variables within the reservoir tank and (2) as a result of losses which occur in piping between the reservoir and the spray bar so In practice, vapor pressure of incoming liquid nitrogen from a reservoir tank will have significant variation in its vapor pressure.
The prior art has attempted to overcome the vapor pressure variation through the use of a "programmed blow-down " and subsequent pressure build-up within the reservoir tank. The blow-down causes a pressure reduction in the tank, enabling an uppermost layer of the liquid nitrogen to boil and absorb heat from the body of the liquid. The blow-down process is inefficient in that gas phase contents are lost and the walls of the tank that are wetted by the gas are cooled down to saturation temperature during the venting process. The walls are then reheated in the pressure rebuilding process consuming additional liquid product.
Subcoolers of various types have been proposed for use in cryogenic freezing operations to achieve temperature control. A subcooler is a temperature reduction/vapor condensing means which delivers a liquid cryogen at its outlet in a subcooled liquid state, i.e., at a pressure higher than its equilibrium vapor pressure at the temperature at which the cryogen exits from the subcooler. U.S. Pat. Nos. 4,296,610 to Davis and 5,079,925 to Maric both disclose prior art subcooler devices. Such subcoolers have a number of limitations. Typically, conventional subcooler designs do not provide a means to closely control the outlet nitrogen temperature and, furthermore, do not provide enough capacity for ordinary freezing operations. Moreover, such subcoolers have generally been set up as independent structures and include complicated piping and tankage.
Accordingly, it is an object of this invention to provide an improved system wherein cryogen may be provided to a use point or consumption means and wherein the cryogen temperature at an outlet is maintained at a constant temperature.
It is another object of this invention to provide an improved subcooler which enables temperature control of a main cryogen feed so as to achieve a constant temperature outlet.
It is yet another object of this invention to provide an improved product refrigeration system wherein a constant inlet cryogen feed is provided to enable efficient refrigeration.